linux-kernel - Re: [PATCH/RFC] arm64: defconfig: Enlarge CMA alignment to 2 MiB

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Date:   Mon, 13 Feb 2017 11:29:27 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Magnus Damm <magnus.damm@...il.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC] arm64: defconfig: Enlarge CMA alignment to 2 MiB

Hi Magnus,

On Wed, Feb 1, 2017 at 2:45 PM, Magnus Damm <magnus.damm@...il.com> wrote:
> On Sat, Jan 28, 2017 at 1:03 AM, Geert Uytterhoeven
> <geert+renesas@...der.be> wrote:
>> Some IOMMUs (e.g. Renesas IPMMU/VMSA) support only page sizes of 4 KiB,
>> 2 MiB, and 1 GiB.
>>
>> With the default setting of CONFIG_CMA_ALIGNMENT = 8, allocations larger
>> than 1 MiB are aligned to a 1 MiB boundary only.  Hence a 2 MiB
>> allocation may not be aligned, leading to a mapping of 512 4 KiB pages.
>>
>> Increase CONFIG_CMA_ALIGNMENT to allow mapping a 2 MiB buffer using a
>> single PTE, decreasing memory usage and TLB pressure.
>>
>> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>> ---
>> Is this useful?
>>
>> Should there instead be different defaults in Kconfig, depending on
>> enabled platform support?
>
> I think there is a dependency on the kernel page size configuration as
> well. In case of 16 KiB or 64 KiB page size configuration other large
> page sizes may be required.

This is about IOMMU pages, not (CPU)MMU pages?

Still, there is indeed some dependency, as CONFIG_CMA_ALIGNMENT is a
shift relative to PAGE_SHIFT (i.e. CONFIG_ARM64_PAGE_SHIFT = 12 in
arm64_defconfig).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds